MCODE question - C=-C-1 MS sets carry in MLDL but not in SDK41



#2

To the MCODE gurus

There seems to be an interesting discrepancy between the behavior of MCODE in the 41 (currently using a HEPAX) and in SDK41. Given that the SDK41 uses the original roms, this surprised me.

Even more surprised am I that the instruction C=-C-1 MS can create a carry with a very benign positive number in C (MS=0). The code below runs as shown in SDK41 and produces for 100 in x-reg the correct value of 71. However when run within an MLDL it produces -29 as the ?NCXQ in line x11F and x124 are not run. If I insert a NOP infront of them the code runs in the MLDL as well, producing the correct result.

Please see the code snippet below (the code does not do anything useful, it’s a preamble to a Prime-Twin generator, my usual way of learning a new language). Any thoughts with regards to

  • How C=-C-1 for a benign positive number can set the carry
  • Difference between SDK41 and MLDL
would be much appreciated.

Thanks so much !

Cheers

Peter

;calculate startvalue for search for prim-twins
;given a number in X, startvalue = 30*Int((X+19)/30) – 19
;implemented as (X+19) – (X+19)Mod30 -19
Add HexCode Command Comment
x107 2A0 SetDec
x108 1A0 A=B=C=0
x109 35C R=12 ;load 19 into C
x10A 050 LD@R 1
x10B 250 LD@R 9
x10C 226 C=C+1 S&X
x10D 070 N=C All ;save for later use
x10E 10E A=C All ;prepare regs for AD2_10
x10F 0F8 Read 3(x)
x110 01D060 ?NCXQ [AD2_10] ;this one works fine
x112 10E A=C All ;A=X+19
x113 158 M=C All ;save in M for later
x114 04E C=0 All ;Load 30 into C to prep for [MOD10]
x115 35C R=12
x116 0D0 LD@R 3
x117 226 C=C+1 S&X
x118 171064 ?NCXQ [MOD10] ;A Mod C
x11A 0EE B<>C ;prep for subtraction via AD2_10
x11B 198 C=M All ;recall X+19
x11C 10E A=C All
x11D 0CE C=B All ;(X+19) Mod 30
x11E 2BE C=-C-1 MS ;This should simply make it negative. But it sets Carry!!
x11F 01D060 ?NCXQ [AD2_10] ;this only works if I have a NOP in front to clear carry when run in MLDL
;it calcs (X+19) – (X+19)Mod30
x121 10E A=C All ;prep to subtract 19
x122 0B0 C=N All ;recall 19 from N
x123 2BE C=-C-1 ;same problem it sets carry when run in MLDL
x124 01D060 ?NCXQ [AD2_10]
x126 0E8 WRIT 3(x) ;save result to x
x127 3E0 RTN

#3

I don't know anything about SDK41, but you should expect to always get the carry flag set as the result of c=-c-1. For subtract instructions, the carry flag indicates a "borrow" out of the leftmost digit. What you're really doing is subtracting from zero, and that will always have a borrow. If you were working on a two-digit field of C that contained 23, after c=-c-1 you should have 76 with a carry.

In the hardware, subtraction is really implemented by addition of a nine's complement (assuming decimal mode), with both the carry-in and carry-out inverted. For the normal add and subtract instructions, the carry-in is zero, but for 0=-c-1 the carry-in is one (before being inverted).


#4

that makes sense! Thanks Eric for helping me out!

I got confused as in Ken's book the instruction C=-c-1 is explained as 'doing the nine's complement' (p26) and I picked up the 'trick' in one of the books but I must have overlooked the NOP before the jump instruction there.. Anyway, thanks a lot again!

Cheers

Peter


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